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When I wrote about the accumulating issue of urban canine waste and what to do about it, many dog owners were quick to denounce my woefully permissive attitude toward cats.

After all, the argument went, if a few irresponsible dog owners don’t clean up after their pooches, what about cats? Nobody cleans up after them, so why do cat owners get off without criticism?

(A small digression: This is a rhetorical method of deflecting attention from the case under discussion. Roman lawyers long ago identified it as the tu quoque — “you, too” — stratagem. It’s based on the logical fallacy that if somebody else offends, the offence being tried is diminished.)

However, just to be on the side of fairness and impartiality, let’s also look at cats and their waste.

Cats, too, contribute to what the academics who study this kind of thing call “fecal loading” in the urban environment.

First, we have to estimate the urban cat population of Vancouver and the Metro region.

Using the formula devised by the American Veterinary Medical Association to calculate urban pet populations, we wind up with an estimate of about 175,000 cats in Vancouver and about 660,000 across Metro.

Next we have to figure out the average daily fecal deposit by a cat. Luckily, there’s a detailed scientific study from 2006 which does precisely that. It found that output from cats averages about 36 grams per cat per day.

This is somewhat less than one tenth the daily average estimated for dogs, which are on average much larger than cats.

Thus, in Vancouver, domestic felines would produce about 6.3 tonnes of fecal matter per day and just over 2,300 tonnes per year compared with 50 tonnes daily and 18,000 tonnes annually for domestic canines. Across Metro the comparison would be about 24 tonnes daily and 8,700 tonnes annually for cats, compared with about 167 tonnes daily and 61,000 tonnes annually for dogs.

But the scientists studying the problem discovered an interesting wrinkle. About 33 per cent of cats were kept permanently indoors and their waste was managed through household litter boxes and disposed of either by flushing or deposit in the garbage.

More accurately, then, Vancouver’s cat population would be depositing outdoors about 4.2 tonnes daily and 1,542 tonnes annually, while across Metro it would amount to about 16 tonnes per day and 5,800 tonnes annually, or about eight per cent of the region-wide accumulation of 69,700 tonnes of pet waste per year.

The math tells us, then, that while cats do contribute to the problem of urban pet waste, it’s on a significantly smaller scale, even though there are more cats than there are dogs.

What about the other aspect of pet waste management that touched a nerve with dog owners — the issue of public health and exposure to pet feces in public areas and where public waterways may be contaminated by run-off?

Here, too, cats contribute to the public health risk but the contribution appears to be significantly smaller. The scientists studying feline fecal loading found, for example, that only 2.5 per cent of E. coli bacteria in the research area traced back to cats. Perhaps this is because cats bury their feces. A study in Seattle, however, traced about 20 per cent of bacterial contamination in run-off back to dogs.

This isn’t to say, though, that there’s no public health footprint from cat feces or that consequences cannot be large.

In California, Toxoplasmo gondii, a protozoan parasite shed only in the feces of cats, caused 16 per cent of the deaths of sea otters near Santa Barbara between 1998 and 2004 and it had infected 38 per cent of the live otters tested. Sea otters in habitats near heavy freshwater outflows were found to be three times as likely to be at risk of infection.

Like dogs, cats shed in their feces a range of bacteria, parasites and viruses that can affect human health, including campylobacter, cat scratch fever, cryptosporidium, giardia, tapeworm, hookworm, ringworm, salmonella and the toxocara worms which can cause blindness, and toxoplasmosis, which is dangerous to pregnant women and people with compromised immune systems.

So there it is, no free ride for the cat fanciers. But nor can dog owners get away with blaming it all on the cats.

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